CN114269959A

CN114269959A - Steel and method for producing same

Info

Publication number: CN114269959A
Application number: CN202080058304.7A
Authority: CN
Inventors: 泉大地; 中岛孝一; 植田圭治
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-08-21
Filing date: 2020-08-18
Publication date: 2022-04-01
Also published as: US20220275489A1; JPWO2021033693A1; JP6947331B2; WO2021033693A1; EP4019657A4; BR112022003041A2; TW202118883A; KR20220041913A; EP4019657A1; MY196520A; TWI732658B

Abstract

The steel of the present invention contains, in mass%, C: 0.100 to 0.700%, Si: 0.05 to 1.00%, Mn: 20.0-40.0%, P: less than or equal to 0.030 percent, S: less than or equal to 0.0050%, Al: 0.01-5.00%, Cr: 0.5-7.0%, N: 0.0050 to 0.0500%, O: less than or equal to 0.0050%, Ti: less than or equal to 0.005% and Nb: 0.005% or less and selected from Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100% and REM: 0.0010 to 0.0200%, has a microstructure having austenite as a base phase, an average crystal grain size of 50 [ mu ] m or less, and a cleanliness of sulfide-based inclusions of less than 1.0%, and has a brittle fracture ratio of less than 5% after a Charpy impact test at-269 ℃ and a yield strength of 400MPa or more.

Description

Steel and method for producing same

Technical Field

The present invention relates to steel suitable for structural steel used in extremely low temperature environments such as liquid helium and liquefied gas, including tanks for storing liquid hydrogen, and a method for manufacturing the same.

Background

When a hot-rolled steel sheet is used for a structure for a liquid hydrogen, liquid helium, or liquefied gas storage tank, the steel sheet has high strength and the use environment is extremely low, and therefore excellent toughness at extremely low temperatures is also required. For example, when a hot-rolled steel sheet is used for a liquid helium storage tank, it is necessary to ensure excellent toughness at a temperature of-269 ℃ or lower, which is the boiling point of helium. If the extremely low temperature toughness of the steel is deteriorated, the safety as a structure for an extremely low temperature storage tank may not be maintained, and therefore, it is strongly required to improve the extremely low temperature toughness of the steel for the use.

In response to this demand, conventionally, austenitic stainless steels that do not exhibit brittleness at extremely low temperatures have been used as the structure of the steel sheet. However, since the alloy cost and the production cost are high, there is a strong demand for a steel material that is inexpensive and has excellent cryogenic temperature toughness.

Therefore, as a new steel material replacing conventional low-temperature steel, for example, patent document 1 proposes that high Ni steel to which Ni as an austenite stabilizing element is added in a large amount is used as structural steel in a-253 ℃. Patent document 1 proposes a technique for ensuring extremely low temperature toughness by controlling the grain size, morphology, and the like of prior austenite.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2018-104792.

Disclosure of Invention

According to the technique described in patent document 1, a high Ni steel excellent in cryogenic temperature toughness can be provided, and from the viewpoint of ensuring cryogenic temperature toughness, the high Ni steel described here must contain not less than 12.5% of Ni, and reduction in material cost is required. Further, in order to secure the austenite phase or the like, it is necessary to perform heat treatment such as reheating quenching, intermediate heat treatment, and tempering, and therefore, there is a problem that the production cost is high.

Accordingly, an object of the present invention is to provide a steel having high strength and excellent cryogenic temperature toughness, which can suppress the material cost and the manufacturing cost. Furthermore, it is an object of the present invention to provide an advantageous method for manufacturing such a steel. The term "high strength" as used herein means a yield strength of 400MPa or more at room temperature, and the term "excellent extremely low temperature toughness" means a brittle fracture ratio of less than 5.0% after Charpy impact test at-196 ℃ and further-269 ℃.

In order to achieve the above object, the inventors have made intensive studies on various factors that determine the composition and structure of a steel sheet, with steel having a relatively large Mn content of 20.0% or more, and have obtained the following findings a to c.

a. Since the austenitic steel contains a large amount of Mn, a large amount of sulfide-based inclusions are present as compared with carbon steel. The sulfide-based inclusions are mainly MnS. Since sulfide-based inclusions are an important factor for the starting point of fracture, when the cleanliness of sulfide-based inclusions after hot rolling and cooling treatment is 1.0% or more, the extremely low temperature toughness deteriorates. Therefore, in order to improve the cryogenic temperature toughness of the steel, it is effective to reduce sulfide-based inclusions.

b. When hot rolling is performed under appropriate conditions, the cleanliness of sulfide inclusions can be suppressed to less than 1.0%, and the heat treatment step is not performed after rolling, whereby the extremely low temperature toughness of steel can be improved, and the production cost can be suppressed.

c. In addition, by performing hot rolling under appropriate conditions, the yield strength of the steel can be improved by providing a high dislocation density and controlling the grain size to an appropriate value.

The present invention has been made in view of the above-described findings, and the gist thereof is as follows.

1. A steel having a composition comprising, in mass%, C: 0.100-0.700%, Si: 0.05% -1.00%, Mn: 20.0% -40.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.01% -5.00%, Cr: 0.5% -7.0%, N: 0.0050 to 0.0500%, O: 0.0050% or less, Ti: 0.005% or less and Nb: 0.005% or less, and further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% -0.0100%, Mg: 0.0005% -0.0100% and REM: more than 1 of 0.0010 to 0.0200 percent, the balance being Fe and inevitable impurities,

has a microstructure containing austenite as a matrix phase, wherein the microstructure has an average crystal grain size of 50 [ mu ] m or less, a cleanliness of sulfide-based inclusions of less than 1.0%, and a brittle fracture ratio of less than 5% after a Charpy impact test at-269 ℃ and a yield strength of 400MPa or more.

2. The steel according to the above (1), wherein the composition further contains, in mass%, a metal selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less and W: 2.0% or less, and 1 or more.

3. A method for producing steel, comprising heating a steel slab having the composition described in the above (1) or (2) to a temperature range of 1100 to 1300 ℃, hot rolling the steel slab, wherein in the hot rolling, in the temperature range of 900 ℃ or higher, the time between passes until the next pass is performed is 200 seconds or less, and the pass reduction (%) of the next pass/the time between passes (seconds) ≥ 0.015 (%/second) is satisfied, finish rolling is performed at a finish rolling temperature of 700 ℃ or higher and less than 900 ℃, and thereafter, cooling treatment is performed at an average cooling rate of 1.0 ℃/s or higher from a temperature of (finish rolling temperature-100 ℃) or higher to a temperature range of 300 to 650 ℃.

Here, the temperatures are the surface temperatures of the steel material or the steel sheet, respectively.

According to the present invention, a steel having high strength and excellent cryogenic toughness can be provided. Therefore, the steel of the present invention greatly contributes to improvement of safety and life of steel structures used in extremely low temperature environments, such as liquid hydrogen, liquid helium, and tanks for liquefied gas storage tanks, and has industrially significant effects. In addition, the production method of the present invention does not cause a reduction in productivity or an increase in production cost, and therefore can provide a method excellent in economy.

Drawings

Fig. 1 is a graph showing the relationship between the average crystal grain size (average grain size) and the yield strength of steel satisfying the composition of the present invention.

FIG. 2 is a graph showing the relationship between the cleanliness of sulfide-based inclusions in steel satisfying the production conditions of the present invention and the brittle fracture ratio at-269 ℃.

Detailed Description

The steel of the present invention will be described in detail below.

[ composition of ingredients ]

First, the composition of the steel of the present invention and the reasons for the limitation thereof will be described. The "%" of the component composition means "% by mass" unless otherwise specified.

C：0.100％～0.700％

C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain this effect, C needs to be contained by 0.100% or more. On the other hand, if the content exceeds 0.700%, Cr carbide is excessively formed, and the very low temperature toughness is lowered. Therefore, the amount of C is 0.100% to 0.700%. The amount of C is preferably 0.200% or more, preferably 0.600% or less, and more preferably 0.200% to 0.600%.

Si：0.05％～1.00％

Si functions as a deoxidizing material, and has an effect of increasing the strength of a steel sheet by solid solution strengthening, as well as being required for steel making. In order to obtain such an effect, Si needs to be contained by 0.05% or more. On the other hand, if it exceeds 1.00%, the non-thermal stress (internal stress) excessively increases, and therefore the very low temperature toughness deteriorates. Therefore, the Si content is 0.05% to 1.00%. The amount of Si is preferably 0.07% or more, preferably 0.80% or less, and more preferably 0.07% to 0.80%.

Mn：20.0％～40.0％

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, Mn is an element that is important for achieving both strength and low-temperature toughness by austenitizing the structure. In order to obtain this effect, Mn needs to be contained by 20.0% or more. On the other hand, if the content exceeds 40.0%, the grain boundary strength decreases, and the very low temperature toughness deteriorates. Therefore, the Mn content is 20.0% to 40.0%. The Mn content is preferably 23.0% or more, preferably 38.0% or less, and more preferably 23.0% to 38.0% or less. The Mn content is more preferably 36.0% or less.

P: less than 0.030%

If the content of P exceeds 0.030%, P excessively segregates in grain boundaries, and therefore the extremely low temperature toughness decreases. Therefore, the upper limit is 0.030%, and the lower limit is preferably as low as possible. Therefore, P is 0.030% or less. Since an excessive reduction in P increases the refining cost and is economically disadvantageous, it is preferably 0.002% or more. The P content is more preferably 0.005% or more, preferably 0.028% or less, more preferably 0.005% to 0.028%, and further preferably 0.024% or less.

S: 0.0050% or less

S deteriorates the extremely low temperature toughness and ductility of the steel sheet, and therefore 0.0050% is set as an upper limit, and preferably as low as possible. Therefore, S is 0.0050% or less. The amount of S is preferably 0.0045% or less. Since an excessive reduction in S increases the refining cost and is economically disadvantageous, the amount of S is preferably 0.0010% or more.

Al：0.01％～5.00％

Al functions as a deoxidizer and is most commonly used in a molten steel deoxidizing step of a steel sheet. In addition, Al contributes to improvement in yield strength and local elongation in a tensile test. In order to obtain such an effect, Al needs to be contained by 0.01% or more. On the other hand, if it exceeds 5.00%, the inclusion is present in a large amount and deteriorates the very low temperature toughness, so that it is 5.00% or less. Therefore, the amount of Al is 0.01% to 5.00%. The amount of Al is preferably 0.02% or more, preferably 4.00% or less, and more preferably 0.02% to 4.00%.

Cr：0.5％～7.0％

Cr is an element effective for improving the extremely low temperature toughness because it improves the grain boundary strength. Cr is an element effective for improving strength. In order to obtain such an effect, Cr needs to be contained by 0.5% or more. On the other hand, if it exceeds 7.0%, the very low temperature toughness is lowered by the formation of Cr carbide. Therefore, the Cr content is 0.5% to 7.0%. The amount of Cr is preferably 1.0% or more, more preferably 1.2% or more, preferably 6.7% or less, more preferably 6.5% or less, more preferably 1.0% to 6.7%, and further preferably 1.2% to 6.5%.

N：0.0050％～0.0500％

N is an austenite stabilizing element and is an element effective for improving the extremely low temperature toughness. In order to obtain such an effect, N needs to be contained by 0.0050% or more. On the other hand, if the content exceeds 0.0500%, the nitride or carbonitride coarsens and the toughness decreases. Therefore, the N content is 0.0050% to 0.0500% or less. The amount of N is preferably 0.0060% or more, preferably 0.0400% or less, more preferably 0.0060% to 0.0400%.

O: 0.0050% or less

O deteriorates the very low temperature toughness due to the formation of oxides. Therefore, O is 0.0050% or less. The O content is preferably 0.0045% or less. Since excessive reduction of O increases refining cost and is economically disadvantageous, the amount of O is preferably 0.0010% or more.

Respectively suppressing the content of Ti and Nb below 0.005%

Ti and Nb form high melting point carbonitrides in steel, and thus excessive content thereof lowers the very low temperature toughness. Ti and Nb are components that are inevitably mixed from raw materials and the like, and in most cases, Ti: more than 0.005% and 0.010% or less and Nb: more than 0.005% and not more than 0.010% are mixed. Therefore, according to the method described later, it is necessary to intentionally limit the mixing amounts of Ti and Nb and to suppress the respective contents of Ti and Nb to 0.005% or less. By suppressing the contents of Ti and Nb to 0.005% or less, respectively, the negative effects of the above-described carbonitrides can be eliminated, and excellent cryogenic temperature toughness and ductility can be ensured. The contents of Ti and Nb are preferably set to 0.003% or less, respectively. Of course, the content of Ti and Nb may be 0% each, and is preferably 0.001% or more each, since it is not preferable to decrease excessively from the viewpoint of steel manufacturing cost.

Is selected from Ca: 0.0005% -0.0100%, Mg: 0.0005% -0.0100%, REM: more than 1 of 0.0010% -0.0200%

Ca. Mg and REM are elements useful for morphological control of inclusions. In the form control of inclusions, the stretched sulfide-based inclusions are referred to as granular inclusions. The ductility and toughness are improved by controlling the morphology of the inclusions. In order to obtain such effects, Ca and Mg are 0.0005% or more, and REM is preferably contained in an amount of 0.0010% or more. On the other hand, if any element is contained in a large amount, the amount of non-metallic inclusions increases, and ductility and toughness may be rather reduced. In addition, it is sometimes economically disadvantageous.

Therefore, each of Ca and Mg is preferably 0.0005% to 0.0100% when Ca and Mg are contained, and 0.0010% to 0.0200% when REM is contained. The amount of Ca is more preferably 0.0010% or more, still more preferably 0.0080% or less, and still more preferably 0.0010% to 0.0080%. The Mg content is more preferably 0.0010% or more, still more preferably 0.0080% or less, and still more preferably 0.0010% to 0.0080%. The REM amount is more preferably 0.0020% or more, more preferably 0.0150% or less, and further preferably 0.0020% to 0.0150%.

REM is a rare earth metal, is a general term for 15 elements of lanthanides plus 17 elements of Y and Sc, and may contain 1 or 2 or more of these elements. The content of REM means the total content of these elements.

In the present invention, the following elements may be contained as necessary in addition to the above-mentioned essential elements for the purpose of further improving the strength and the cryogenic temperature toughness.

Is selected from Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less of 1 or more

Cu and Ni: respectively less than 1.0%

Cu and Ni are elements that not only increase the strength of the steel sheet by solid solution strengthening, but also improve the ease of dislocation and also improve low-temperature toughness. In order to obtain such effects, Cu and Ni are preferably contained by 0.01% or more, and more preferably 0.03% or more. On the other hand, if the content exceeds 1.0%, the surface properties are deteriorated during rolling, and the production cost is also increased. Therefore, when these alloying elements are contained, the content is preferably 1.00% or less, more preferably 0.70% or less. The Cu content and the Ni content are preferably 0.03% to 0.70%, more preferably 0.50% or less.

Mo, V, W: respectively less than 2.0%

Mo, V and W contribute to the stabilization of austenite and to the improvement of the strength of steel. In order to obtain such effects, Mo, V, and W are preferably contained at 0.001% or more, and more preferably at 0.003% or more. On the other hand, if the content exceeds 2.0%, coarse carbonitrides are generated, which become starting points of fracture and further, the production cost is suppressed. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less, and more preferably 1.7% or less. The respective amounts of Mo, V and W are more preferably 0.003% to 1.7%, still more preferably 1.5% or less.

The remainder of the composition other than the above components is a composition of components including iron and inevitable impurities. The inevitable impurities include H, B and the like, and the total amount is preferably 0.01% or less.

[ tissue ]

Microstructure with austenite as base phase

When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material is likely to cause brittle fracture in an extremely low temperature environment, and therefore, the steel material is not suitable for use in an extremely low temperature environment. Therefore, when used under an assumed extremely low temperature environment, the base phase of the steel material preferably has an austenite structure having a face-centered cubic structure (fcc). The phrase "austenite is used as a base phase" means that the austenite phase is 90% or more, and more preferably 95% or more, in terms of area ratio. The remainder other than the austenite phase is a ferrite phase or a martensite phase.

The average crystal grain diameter of the microstructure is 50 μm or less

As a result of examining the relationship between the average crystal grain size and the yield stress in the tensile test, as shown in fig. 1, in the steel having the composition of the present invention, if the average crystal grain size is 50 μm or less, it is determined that the yield stress may be 400MPa or more.

Here, the crystal grains in the present specification mainly refer to austenite crystal grains, and the average crystal grain diameter thereof is determined as an average value of 100 randomly selected crystal grains in an image taken at 200 times using an optical microscope, calculated as a circle-equivalent diameter.

The cleanliness of sulfide-based inclusions in the microstructure is less than 1.0%

As a result of examining the relationship between the cleanliness of sulfide-based inclusions and the brittle fracture ratio in charpy impact test, as shown in fig. 2, it was determined that if the cleanliness of sulfide-based inclusions is less than 1.0% in the steel satisfying the production conditions of the present invention, the brittle fracture ratio can be made less than 5%.

Here, the cleanliness in the present specification can be obtained in the examples described later.

The above average crystal particle diameter: cleanliness of sulfide-based inclusions of 50 μm or less: less than 1.0% can be achieved by hot rolling under the conditions described below under the above-described composition.

The steel according to the present invention can be produced by melting molten steel having the above-described composition by a known melting method such as a converter or an electric furnace. In addition, refining can be performed 2 times in a vacuum degassing furnace. In this case, in order to limit Ti and Nb which inhibit the appropriate structure control to the above-mentioned ranges, it is preferable to avoid inevitable mixing from raw materials and the like and to take measures to reduce these contents. For example, these alloys can be discharged by thickening the slab by reducing the basicity of the slab in the refining stage, and the Ti and Nb concentrations of the final slab product can be reduced. Further, a method of blowing oxygen and oxidizing the blown oxygen, and separating an alloy of Ti and Nb on the surface during the reflow may be used. Thereafter, a steel material such as a slab having a predetermined size is formed by a known casting method such as a continuous casting method or a cast-cogging method.

The manufacturing conditions for manufacturing the steel material into a steel material having excellent cryogenic toughness are specified.

In order to obtain a steel having the above physical properties, it is important to heat a billet (steel stock) to a temperature range of 1100 to 1300 ℃, and then perform the following rolling passes within 200 seconds so as to satisfy a pass reduction (%) and a time(s) between passes of not less than 0.015%/s in hot rolling in a temperature range of 900 ℃ or higher, and further perform hot rolling at a finish rolling temperature of not less than 700 ℃ and less than 900 ℃ as a finish rolling. The temperature here means the surface temperature of the steel material.

[ heating temperature of the billet: 1100 ℃ -1300℃)

In order to exhibit the above-described effects of Mn, it is important to diffuse Mn in steel. That is, the heating temperature of the steel stock before hot rolling is 1100 ℃ or higher in order to diffuse Mn by hot rolling. On the other hand, if it exceeds 1300 ℃, the melting of the steel may be started, so the upper limit of the heating temperature is 1300 ℃. The heating temperature of the billet is preferably 1130 ℃ or higher, preferably 1270 ℃ or lower, and more preferably 1130 to 1270 ℃.

[ hot rolling at 900 ℃ or higher: the time between passes is within 200 seconds, and the pass reduction ratio (%)/the time between passes(s) ≥ 0.015 (%/s) ]

After heating the steel billet by the above method, hot rolling is performed. In particular, in the rolling in the temperature range of 900 ℃ or higher, it is important to perform the next rolling pass within 200 seconds as the time between passes. Therefore, if the steel blank is maintained in the temperature range for a long time in the rolling at 900 ℃ or higher, the crystal grains start to grow and coarsen. The rolling pass interval (inter-pass time) is preferably 150 seconds or less, and more preferably 100 seconds or less. The lower limit of the inter-pass time is not particularly limited, and the inter-pass time is preferably set to an interval of at least 5 seconds if the processing in the actual process is taken into consideration. The upper limit of the hot rolling temperature is not particularly limited, but is preferably 1250 ℃. Here, when a plurality of inter-pass times exist in the temperature range of 900 ℃ or higher (that is, when rolling is performed at least 3 times in the temperature range of 900 ℃ or higher), the maximum time (maximum value) among the plurality of inter-pass times may be 200 seconds or less.

In addition, for rolling in a temperature range of 900 ℃ or higher, it is necessary to satisfy the pass reduction (%) and the time (sec) between passes of not less than 0.015 (%/sec) in each of the second and subsequent passes. Therefore, austenite is recrystallized finely, and the growth of crystal grains after the recrystallization is completed can be suppressed, and the generation of coarse crystal grains can be suppressed reliably. Here, when a plurality of pass reductions/inter-pass times exist in the temperature region of 900 ℃ or higher, the minimum value of the pass reduction/inter-pass time may be 0.015 (%/second) or more. The pass reduction/time between passes is preferably 0.020 (%/second) or more.

[ finishing temperature: 700 ℃ or higher and less than 900 ℃)

The finish rolling requires 1 or more passes at a finish rolling temperature of 700 ℃ or higher and less than 900 ℃. That is, the crystal grains can be refined by performing rolling at less than 900 ℃ for 1 pass or more. In addition, when finish rolling is performed in a temperature range of 900 ℃ or higher, the crystal grain size becomes excessively coarse, and the desired yield strength cannot be obtained. Therefore, it is preferable to perform finish rolling in 1 pass or more at less than 900 ℃. The finish rolling temperature is preferably 890 ℃ or lower, more preferably 880 ℃ or lower. On the other hand, if the finish rolling temperature is less than 700 ℃, the very low temperature toughness deteriorates and is set to 700 ℃ or higher. The finishing temperature is preferably 750 ℃ or higher. The reduction ratio of the finish rolling is preferably 10% or more for 1 pass.

The thickness of the plate after the finish rolling is not particularly limited, but is preferably 6 to 30mm in consideration of the use as a structure for an extremely low temperature storage tank.

[ average cooling rate from a temperature of not less than (finish rolling temperature-100 ℃) to a temperature range of 300 ℃ to 650 ℃: 1.0 ℃/s or more

After the hot rolling is finished, cooling treatment is performed at a high cooling rate. If the cooling rate of the steel sheet after hot rolling is lowered, the formation of carbide is promoted, resulting in deterioration of the very low temperature toughness. The formation of these carbides is suppressed by cooling at an average cooling rate of 1.0 ℃/s or more in a temperature range from (finishing temperature-100 ℃) or more to 300 ℃ to 650 ℃. The reason why the cooling temperature region is set to this temperature region is to suppress precipitation of carbide, and particularly, the reason why the cooling start temperature is set to (finish rolling temperature-100 ℃) or higher is to promote precipitation of carbide when the cooling start temperature is lower than (finish rolling temperature-100 ℃) after finish rolling. In addition, if excessive cooling is performed, the steel sheet deforms, and productivity is reduced. Therefore, the upper limit of the cooling start temperature is preferably 900 ℃. The upper limit of the average cooling rate is not particularly limited, but is preferably 200 ℃/s or less. In particular, it is preferable to air-cool a steel material having a plate thickness of less than 10 mm.

The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples.

Billets (slabs) having the composition shown in table 1 were produced by the converter-ladle refining-continuous casting method. The obtained slabs were hot-rolled under the conditions shown in Table 2 to form steel sheets having a thickness of 6 to 30 mm. The obtained steel sheet was subjected to the following evaluation of texture and mechanical properties such as tensile properties and cryogenic temperature toughness.

In table 2, "inter-pass time in hot rolling at 900 ℃ or higher" indicates the longest time (maximum value) among the plurality of inter-pass times, and "pass reduction/inter-pass time" indicates the smallest value among the plurality of pass reduction/inter-pass times. The "finishing temperature at the time of finish rolling" represents a finish rolling finishing temperature.

[ Table 2]

The same underlines indicate the scope of the invention

(1) Tissue evaluation

Area fraction of austenite phase

The area ratio of each phase of the microstructure was determined from a phase map analyzed by back scattered electron diffraction (EBSD). At the 1/2 th position of the thickness of the obtained steel sheet, a test piece for EBSD analysis was sampled from a cross section parallel to the rolling direction, and EBSD analysis was performed at a measurement pitch of 0.3 μm in a field of view of 500. mu. m.times.200. mu.m, and the value shown in the phase diagram was defined as the area ratio.

The area ratio of the austenite phase was 90% or more in all of the invention examples and comparative examples, and it was confirmed that the matrix phase was austenite.

Average crystal grain size

The steel sheet after cooling treatment after finish rolling was polished in the rolling direction cross section, 100 crystal grains were randomly selected from an image of the steel sheet at a position of 1/2 mm in plate thickness taken at a magnification of 200 times using an optical microscope, and the average crystal grain size was determined from the equivalent circle diameter.

Cleanliness of sulfide-based inclusions

The sheet thickness 1/2 position on the polished surface of the section in the rolling direction was observed in any 60 visual fields at a magnification of 400 times using a microscope, and the cleanliness d (%) was calculated using the following formula, with respect to group a among inclusions, on the steel sheet subjected to the cooling treatment after the finish rolling, based on the provisions of JIS G0555 (2003).

d＝(n/p×f)×100

p: total number of grid points within field of view, f: number of fields of view, n: number of grid point centers occupied by inclusions of f fields

(2) Evaluation of tensile Properties

From the obtained steel sheets, tensile test specimens of JIS4 were prepared for steel sheets having a thickness of more than 15mm, and round bar tensile test pieces having a parallel portion diameter of 6mm and an inter-gauge distance of 25mm were prepared for steel sheets having a thickness of 15mm or less, and tensile test was conducted to examine tensile test characteristics (yield strength, tensile strength, total elongation). In the present invention, the tensile properties are judged to be excellent when the yield strength is 400MPa or more.

(3) Evaluation of extremely Low temperature toughness

From the direction parallel to the rolling direction at the 1/2-point of the thickness of each steel sheet exceeding 10mm, Charpy V notch test pieces were sampled in accordance with the JIS Z2242 (2005) and each steel sheet was subjected to 3 Charpy impact tests at-196 ℃ and-269 ℃. Each steel sheet having a thickness of less than 10mm was subjected to 3 Charpy impact tests at-196 ℃ and-269 ℃ using a Charpy V notch test piece having a size of 5mm from a direction parallel to the rolling direction at a position having a thickness of 1/2 according to the specification of JIS Z2242 (2005). The brittle fracture ratio was determined by visual observation. The case where the brittle fracture ratio is less than 5% was judged to be excellent in very low temperature toughness. The Charpy impact test at-269 ℃ was carried out by encapsulating the test piece and flowing liquid helium.

Reference 1: T.Ogata, K.Hiraga, K.Nagai, and K.Ishikawa Tetsu-to-Haganie, 69(1983),641.

The results obtained by the above evaluations (1) to (3) are shown in table 3.

[ Table 3]

The same underlines indicate the scope of the invention

It was confirmed that the steel according to the present invention satisfies the above-mentioned target properties (yield strength of 400MPa or more, and brittle fracture ratio after Charpy impact test of less than 5%). On the other hand, the comparative examples outside the scope of the present invention are those in which any one or more of the yield strength and the brittle fracture ratio cannot satisfy the above-mentioned target performance.

Claims

1. A steel having a composition comprising, in mass%, C: 0.100-0.700%, Si: 0.05% -1.00%, Mn: 20.0% -40.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.01% -5.00%, Cr: 0.5% -7.0%, N: 0.0050 to 0.0500%, O: 0.0050% or less, Ti: 0.005% or less and Nb: the content of the active carbon is less than 0.005%,

further contains, in mass%, a component selected from the group consisting of Ca: 0.0005% -0.0100%, Mg: 0.0005% -0.0100% and REM: more than 1 of 0.0010 to 0.0200 percent, the balance being Fe and inevitable impurities,

has a microstructure with austenite as a base phase,

the average crystal grain size in the microstructure is less than 50 μm and the cleanliness of sulfide inclusions is less than 1.0%,

the yield strength is more than 400MPa, and the brittle fracture ratio after a Charpy impact test at-269 ℃ is less than 5 percent.

2. The steel according to claim 1, wherein the composition further contains, in mass%, a metal selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less and W: 2.0% or less, and 1 or more.

3. A method for producing steel by heating a steel slab having the composition of claim 1 or 2 to a temperature of 1100 to 1300 ℃ and hot rolling the heated slab,

in the hot rolling, in a temperature range of 900 ℃ or higher, the time between passes until the next rolling pass is performed is 200 seconds or less, and the pass reduction/the time between passes of the next rolling pass is 0.015%/second or more,

finish rolling is carried out at a finish rolling temperature of 700 ℃ or more and less than 900 ℃,

then, a cooling treatment is performed at an average cooling rate of 1.0 ℃/s or more in a temperature range from a temperature of not less than (finish rolling temperature-100 ℃) to a temperature of 300 ℃ to 650 ℃.